Quoting Bonhoeffer #7

What really impressed me was not just the face that he surpassed almost all of us in theological knowledge and capacity; but what passionately attracted me to Bonhoeffer was the perception that here was a man who did not only learn and gather in the verba and scripta of some master, but one who thought independently and already knew what he wanted and wanted what he knew. I had the experience (for me it was something alarming and magnificently new!) of hearing a young fair-haired student contradict the revered historian, his Excellency von Harnack answered, but the student contradicted again and again [Metaxas in Bonhoeffer, 59].

Later Metaxas writes,

Besides Harnack, three other Berlin professors had decided influence on Bonhoeffer. They were Karl Holl, who was perhaps the greatest Luther scholar of that generation; Reinhold Seeberg, who specialized in systematic theology, and under whom Bonhoeffer wrote his doctoral thesis; and Adolf Deissman, who was Bonhoeffer’s introduction to the ecumenical movement, which would play such an important role in his life and provide the means by which he became involved in the conspiracy against Hitler. But there was another theologian who had a greater influence on Bonhoeffer than any of these, and whom he would revere and respect as much as anyone in his lifetime, who would even become a mentor and a friend. This was Karl Barth of Göttingen [Metaxas in Bonhoeffer, 60].